The oddity of red colour development in pears

We know we shouldn’t compare apples and oranges, but what about apples and pears? With red colour development in pears, it is best not to compare these two fruits. By Wiehann Steyn.

Red colour development in pears is an anomaly.

Producers need to be familiar with the factors that influence red colour development in pears to maximise profits. Until the research funded by Hortgro, little was known about this topic. The assumption was that red colour development in pears follows the same pattern and is affected by the same factors as apples.

However, this isn’t the case, and pears have their own set of rules for red colour development.

There are two colour-related factors that play a role in whether a blushed pear is lucrative — the presence and extent of red blush. Both factors fluctuate between seasons and cultivars.

In apples, and many other fruit kinds, poor colour at harvest is due to low levels of anthocyanin — the red pigment. Poor light exposure and lack of low temperature before harvest are to blame for this as these two elements induce pigment production.

Local research showed that pear red colour development peaks midway between flowering and harvest. Thereafter, pigment concentrations decrease and the red colour fades towards harvest. This is due to decreased pigment synthesis, natural turnover, dilution as the fruit increase in size, and degradation at high temperatures.

The role of light

Light has opposing effects on pear red colour. It is required for anthocyanin production, but also contributes to red colour loss through pigment degradation. When unfavourable temperatures — or other factors — limit anthocyanin synthesis, light may contribute more to degradation than to synthesis, giving rise to colour loss.

Apple peel’s ability to produce anthocyanin increases with maturity. At this stage, anthocyanin production considerably exceeds degradation, and leads to reddening of the peel. But pears have the opposite occurring — anthocyanin production in pear peel decreases towards harvest, light’s contribution to degradation dominates, and the fruit loses red colour.

The role of temperature

Both apples and pears benefit from low temperatures when it comes to red colour.

Low temperatures increase red pigment accumulation while high temperatures decrease red pigment accumulation. Red colour in all apple cultivars seems to benefit from night temperatures below 15°C — even lower temperatures are required by Cripps Pink.

The red colour of Rosemarie and possibly also Forelle pears seems to benefit from low temperatures, while Bon Rouge and Flamingo were unresponsive. No work has been done on Cheeky red colour development.

When light and high temperature team up, it causes red colour loss in fruit. Pigment loss in red apple and pear peel speeds up with increasing temperature — on a hot day, fruit peel temperature may be 15°C higher than air temperature. Just one day at 30°C can cause substantial colour loss in Rosemarie pears and Cripps Pink apples.

The interplay between maximum pigment levels and pigment loss towards harvest

Full-red cultivars accumulate much more red pigment than blushed cultivars.

Toward harvest, the fruit of full-red cultivars show less red colour fading than blushed cultivars. This is due to colour buffering at high pigment levels. Colour buffering means that if a fruit contains lots of pigment, it can lose a large amount of that pigment without the colour changing much. But, when a fruit contains low levels of the pigment, even a small loss of pigment can cause a marked change in colour.

Rosemarie pears can change from red to green over the course of a few consecutive hot days.

Blushed cultivars show considerable fluctuation in redness between seasons due to their susceptibility to colour change. Based on this knowledge, the ARC — when breeding for stable red colour — was advised to avoid pears with low pigment concentrations, and pears that strongly respond to temperature in terms of red colour development.